It is common practice in modern construction to provide a system for heating or cooling or ventilating a structure. Most installations utilize moving air for one or more of these functions, and it is common to either heat or cool the regions of a building to be temperature controlled with a system of ducts to transport temperature controlled air from a central heating or cooling source to the various regions to be heated or cooled.
In most recently constructed houses, the central heating or cooling source is either a hot air furnace or a coolant-to-air heat exchanger of a central cooling system, and in climates requiring both heating and cooling, both may be employed. Often, the central heating or cooling source is disposed in a basement of the house, or in a pit disposed in the ground beneath a ground-level floor of the house. The air from the source is transported under pressure to a plurality of register heads. The most economical way to transport the air flow in such structures is through ducts located beneath the floors, in the walls and above the ceilings of the structures.
A common practice in the United States is to construct walls using equally spaced vertical studs which support a layer of material on each of the opposite sides of the studs, such as plaster, plaster board, fiber board, particle board, or the like. This construction provides open channels between the studs which are suitable for accommodating ducts for transporting air in heating or cooling systems. Because the studs are generally 4 or 6 inches thick and disposed on 16 inch centers, the cross section of the space between adjacent studs is rectangular, and with 4 inch studs, the maximum usable cross section of the wall channel is about 14 by 31/2 inches.
In order to optimize the air flow through a duct in such a wall, substantially rigid rectangular ducts constructed of sheet metal have been employed. Such ducts are available commercially in 31/2 inch thicknesses and various widths ranging to 14 inches, but the length of available sections are relative short, namely 24, 48, 60, 96 and 100 inches. The sections must be connected together to provide the proper length of duct for each installation, and since the duct is rigid, fittings such as elbows, and angles constructed of sheet metal must be used to provide bends between sections. The sheet metal ducts and fittings are relatively costly and require significant labor to install. Further, the installation must be skillfully completed to assure against leaks, vibration and other noises. Hence, an installation constructed in this manner is relatively costly.
Installations between adjacent studs have also been made using rigid oval sheet metal duct which reduces air flow noise and permits the use of duct with cross sections approaching those of rectangular duct. Spiral-Helix, Inc. of Buffalo Grove, Ill. is marketing a machine for converting rigid-cylindrical sheet metal pipe to oval pipe by stretching the cylindrical pipe from the interior thereof to assume a flat oval shape. Such rigid oval pipe may be installed in the channels between studs in walls and will provide air moving capacity under these conditions significantly greater than a cylindrical duct of the diameter of the studs and approaching the air moving capacity of rectangular duct, but it, like rigid rectangular duct, requires skilled installation and sheet metal fittings to achieve bends, thus retaining the cost disadvantages of rectangular duct.
Further, all rigid duct has the disadvantage of requiring relatively large cartons for storage and shipment, and in order to reduce the size of the cartons, such rigid duct is constructed in relatively short sections. The large cartons add to the cost of storage and shipment, and the short sections add to the cost of installation.
The inventor has recognized that a reduction in the cost of materials and installation could be achieved by using flexible oval or rectangular duct, but such a duct is not commercially available. U.S. Pat. No. 3,216,459 granted on Nov. 9, 1965 to C. A. Schroeder and O. L. Clark entitled Flexible Insulated Duct does disclose a generally rectangular flexible duct which comprises an inner generally rectangularly shaped wire coil surrounded by one or more layers of thermal insulating material and an outer plastic sleeve. The duct of this patent does not have a flexible inner plastic tube which is supported by the wire coil. Further, the duct of the patent lacks a vapor barrier which is generally required in modern heating, cooling or ventilating systems. In addition, the duct of this patent is limited in length to the length of the collapsible mandrel used in its production.